Termite

Termite, (order Isoptera), any of a group of cellulose-eating insects, the social system of which shows remarkable parallels with those of ants and bees, although it has evolved independently. Even though termites are not closely related to ants, they are sometimes referred to as white ants. Phylogenetic studies have shown that the closest relative to the termite is the cockroach; for this reason termites are sometimes placed in the order Dictyoptera, which also contains the mantids.

General features

Distribution and abundance

Termites, which number about 2,750 species, are distributed widely, reaching their greatest abundance in numbers and species in tropical rainforests around the world (seevideo). In North America termites are found as far north as Vancouver, British Columbia (Zootermopsis), on the Pacific coast, and Maine and eastern Canada (Reticulitermes) on the Atlantic coast. In Europe the northern limit of natural distribution is reached by Reticulitermes lucifugus on the Atlantic coast of France, although an introduced species, Reticulitermes flavipes, occurs as far north as Hamburg, Germany. The known European species of termites have a predominantly Mediterranean distribution and do not occur naturally in Great Britain, Scandinavia, Switzerland, Germany, or northern Russia. In the Far East Reticulitermes speratus ranges as far north as South Korea, Peking, and northern Japan. Termites occur also in the Cape region of South Africa, Australia, Tasmania, and New Zealand.

A termite nest in a tropical rainforest.Encyclopædia Britannica, Inc.

In addition to naturally occurring termites, many species have been inadvertently transported by humans from their native habitats to new parts of the world. Termites, particularly Cryptotermes and Coptotermes, have been accidentally transported in wooden articles such as shipping crates, boat timbers, lumber, and furniture. Because dry-wood termites (e.g., Cryptotermes species) live in small colonies in wood and tolerate long periods of dryness, they can survive in seasoned wood and furniture and can easily be transported over long distances. Members of the family Rhinotermitidae (e.g., Coptotermes) require access to moisture and cannot survive prolonged dry periods. Coptotermes formosanus, widely distributed in Japan, Taiwan, and South China, has been introduced into Sri Lanka (Ceylon), the Pacific islands, South Africa, East Africa, Hawaii, California, and the southern United States. C. formosanus is unusual for the family in that it can survive without direct soil contact as long as a moisture source is present. In the United States the species has been found to have well-established colonies in the upper reaches of buildings, using small leaks in the roof as a moisture source. A termite native to the United States, Reticulitermes flavipes, was found in the hothouses of the Royal Palace in Schönbrunn, in Vienna, and the species was reported and described in that location before it was discovered in the United States. The termites presumably had been shipped from North America in wooden containers of decorative potted plants.

Importance

Termites are important in two ways. They are destructive when they feed upon, and often destroy, wooden structures or vegetable matter valuable to humans. Introduced species, because they are not so well equipped as native species to adapt to changes in their new environments, tend to seek shelter in protected, man-made environments such as buildings and thus are likely to become the most serious pests, causing significant damage to houses and wooden furnishings. Some termites feed on living plant materials and can become serious crop pests. Termites are also extremely beneficial in that they help to convert plant cellulose into substances that can be recycled into the ecosystem to support new growth.

Although only about 10 percent of the 2,750 known termite species have been reported as pests, many of these cause severe and extremely costly damage. For effective control, it is essential to determine whether the pest is a subterranean or a wood-dwelling species, as treatment methods differ.

Subterranean termites are dependent on contact with soil moisture and normally reach the wood in man-made structures through the foundations. The most common traditional control used around a structure is to flood a shallow trench with an insecticide and cover it with soil. Insecticides also are useful around cracks and crevices in foundations. A recent development has been to establish permanent monitoring stations around the perimeter of a foundation. When termites are discovered in a station, the wooden “bait” is replaced with a cellulose material containing a chitin inhibitor that is consumed by the foraging workers and returned to the colony and fed to others. This material disrupts normal formation of the exoskeleton (cuticle) of molting workers (the only caste capable of molting), resulting in death of the workers and, eventually, the rest of the colony. Construction and design practices that can prevent the initial entry of subterranean termites into a structure include the use of pressure-treated wood, treated concrete foundation blocks, and reinforced concrete foundations that extend at least 15.2 cm (6 inches) above the ground and have no cracks or contact with any outside wood. Removal of scrap lumber from building sites will also reduce the termite population.

Dry-wood termites nest in the wood on which they feed and do not invade a structure from the soil. Because their colonies are within the structure, they are difficult to control. Preventive measures include the use of chemically treated wood in building construction and the use of paint or other durable finish to seal cracks in wood surfaces. Fumigation is the most effective method for eliminating a dry-wood termite infestation. Another method is to place insecticide into small holes drilled into galleries of infested wood.

Natural history

The termite society, or colony, is a highly organized and integrated unit. There is a caste system with division of labour based on the colony members’ structure, function, and behaviour. The major castes in the colony are the reproductive, soldier, and worker castes. Soldiers and workers are sterile and may be male or female. The functional reproductives are of two types, referred to as primary and secondary, or supplementary.

Mechanisms controlling differentiation of termites into castes are not understood fully. It is known that all nymphs are genetically identical at hatching and that all could develop into any of the three major castes.

The number of individuals in each caste in a colony is closely regulated. Normally there are one pair of reproductives and a set ratio of soldiers to workers and nymphs. If members of any caste are lost, additional members of that caste develop from nymphs to restore the balance. Conversely, if overproduction of one caste occurs, selective cannibalism restores the balance.

Chemical substances such as pheromones and hormones play a role in differentiation, production, and regulation of castes. Both reproductive and soldier castes secrete a pheromone (chemical signal) that is transmitted through food sharing (trophallaxis) and grooming to other members of the colony and inhibits development of reproductives or soldiers. If the caste balance of the colony is upset, some undifferentiated nymphs do not receive the “pheromone message” and thus develop into reproductives or soldiers, thereby restoring the balance.

This inhibition theory has been confirmed by experiments with supplementary reproductive development in Kalotermes and Zootermopsis.

Pheromones may act to control caste differentiation through hormonal action, but it is not clear how this mechanism works. Activation of the corpora allata near the brain may result in release of a hormone, distinct from the juvenile hormone, that causes differentiation of a nymph into a soldier. Similarly, activity of a molt gland may be responsible for differentiation of the reproductive caste. In termites, therefore, hormones not only control molting and metamorphosis, as in other insects, but may also play a role in caste differentiation.

A new termite colony normally is founded by dispersion of winged adults (alates), which usually develop in a mature colony during certain seasons of the year. After molting into winged adults, alates group themselves in special chambers near the periphery of the nest for several days or weeks. Emergence and flight of alates is usually associated with high atmospheric humidity in combination with temperature, climatic, and seasonal factors that vary with the species. In some species one emergence a year may occur; in others there may be many successive flights.

Workers prepare tunnels to the surface, open exit holes, and sometimes construct launching platforms prior to emergence of the alates. During emergence, the soldiers guard the exit holes, not only to prevent entry of enemies but also to prevent alates from re-entering the nest. At the time of emergence the alates, which normally avoid light, become attracted to it and fly out of the nest. They are weak fliers and, unless carried by the wind, descend within several hundred metres of the original colony. The flight, commonly called a nuptial or mating flight, is simply dispersal. Mating occurs after the flight. Swarming from many colonies occurs simultaneously in a given area and may be synchronized closely in areas separated by hundreds of miles. An advantage of synchronization might be intercolony mating.

Shortly after the alates land, they shed their wings, leaving only the base of the wing scale attached to the thorax. During a short courtship, in which the female raises her abdomen and emits a sex attractant, the pair moves off in tandem (pairing), with the male following closely behind the female. The couple then seek a nesting site in a crevice or dig a hole in wood or soil that has been softened by rain and seal the hole with their own fecal matter. Copulation takes place only after the establishment of this nuptial chamber. During copulation, which occurs intermittently throughout the lives of the king and queen, sperm are transferred and stored in the spermatheca of the female. Since the male has no external copulatory organ, sperm are released through a median pore on the ninth sternite, or abdominal plate.

After copulation the first batch of eggs, usually few in number, is laid. In two to five years, as the colony matures, the egg-laying capacity of the queen increases as her ovaries and fat bodies develop and her abdomen enlarges (a process called physogastry). Physogastric queens in more advanced termite families (e.g., Termitidae, especially Macrotermes and Odontotermes) may become 11 cm (4.3 inches) long. The queen becomes an “egg-laying machine” and may produce as many as 36,000 eggs a day for many years. The king is 1 to 2 cm (0.4 to 0.8 inch) long. In temperate regions egg laying stops or slows during the winter months, while in tropical species it continues throughout the year.

The first young nymphs develop into workers or pseudergates and soldiers. Only after the colonies are mature do winged adults develop. During the initial stages of colony formation, the reproductives feed the young and tend the nest. However, as the colony becomes established, the young nymphs perform these duties.

Primitive termite families have small colonies containing hundreds to thousands of individuals. More advanced families (e.g., Rhinotermitidae, Termitidae) have colonies that may number in the thousands to millions of individuals, with all members produced by the single reproductive pair. Workers and soldiers may live two to five years. The primary king and queen in higher termite families may live 60 to 70 years. The entire colony may exist for many years in species that replace the primary king and queen with secondary reproductives.

Other colonizing methods

Sometimes new colonies are formed by budding, the division or accidental separation of part of a colony from an original nest. When this occurs, supplementary reproductives take over as the reproductive pair. Another method of colony formation is sociotomy, or social fragmentation. In this situation, workers, soldiers, and nymphs migrate to a new nesting site, and this fragment of the original colony develops supplementary reproductives. Sometimes an original reproductive pair joins a migrating group.

Nests

Internal features

Since termites have a soft cuticle and are easily desiccated, they live in nests that are warm, damp, dark, and sealed from the outside environment. These nests are constructed by workers or old nymphs. The high relative humidity in the interior of the nest (90 to 99 percent) probably is maintained in part by water production resulting from metabolic processes of individual termites. The temperature inside the nest generally is higher than that of the outside environment. In addition to providing an optimum microclimate, the nest provides shelter and protection against predators.

Since the anaerobic protozoans—which are necessary for cellulose digestion and live in the hindguts of primitive termites—cannot tolerate high concentrations of oxygen, these termites have developed a tolerance for high concentrations of carbon dioxide. In some species, this may be as high as 3 percent. However, ventilation must occur in the nest and is often facilitated by nest architecture. For example, the subterranean nests of Apicotermes have an elaborate system of ventilation pores. Convection currents and diffusion through the nest wall also provide ventilation in large nests.

Nest types

The family Kalotermitidae and the subfamily Termopsinae (family Hodotermitidae) make their nests in the wood on which they feed. These termites excavate irregular networks of galleries with no external openings, except the temporary ones created during swarming. The nest galleries have partitions made of fecal matter and are lined or coated with plaster made of fecal matter. The Kalotermitidae live in the sound wood of stumps and branches of trees. Examples are Neotermes tectonae, which lives in and attacks teak trees in Java, and Cryptotermes, which bores into trees and furniture in various parts of the world. The Termopsinae live in damp rotten logs. Although true wood dwellers never invade soil, and their nests have no soil connections, all other termites are basically subterranean, building their nests either in soil or with soil connections and exploiting food sources away from the nest.

Many species of Rhinotermitidae build nests in wood that is buried in damp soil and from which a diffused network of tunnels to food sources may radiate into the soil or above the ground in the form of covered runways. Other termites build a diffused subterranean nest with many chambers or pockets in soil and a network of galleries.

Many termites build discrete and concentrated nests. Some nests rise partly above the ground as mounds or hills, whereas others are totally underground or arboreal. Dirt, particles of fine clay, or chewed wood glued together with saliva or excreta are used to build nests. During nest construction a termite deposits fecal matter to cement particles in place.

Arboreal nests are ovoid structures built of “carton” (a mixture of fecal matter and wood fragments), which resembles cardboard or papier-mâché. Carton may be papery and fragile, or woody and very hard. The inside of an arboreal nest consists of horizontal layers of cells, with the queen occupying a special compartment near the centre. The nests always maintain connections with the ground through covered runways.

The large termite mounds, or hills, which are a prominent landscape feature in the tropics, may be domelike or conical. Some have chimneys and pinnacles. Longitudinal and horizontal chambers and galleries comprise the interior. Generally the outer wall is constructed of hard soil material, distinct from the internal central portion (or nursery), which is composed of softer carton material. In northern Australia Amitermes meridionalis builds wedge-shaped mounds, called compass or magnetic mounds, that are 3 to 4 metres (9.8 to 13.1 feet) high, 2.5 metres (8.1 feet) wide, and 1 metre (3.2 feet) thick at the base. The long axis is always directed north-south, and the broad side faces east-west, an orientation that probably functions to help regulate temperature. Spectacular mounds are built by fungus-growing termites in Indomalaya and Africa. Mounds of some African Macrotermes species reach a height of 8 to 9 metres (26.2 to 29.5 feet) and have pinnacles, chimneys, and ridges on their outer walls. Such mounds are built of fine particles of clay glued together by saliva to form an exceedingly hard substance. Inside the mounds are honeycomb-like structures on which the fungus grows.

Many termite nests harbour various other invertebrates as guests (e.g., beetles, flies, bugs, caterpillars, millipedes). Some, called termitophiles, are unable to survive independent of their termite hosts. True termitophiles actually have evolved with their hosts and are species specific. Some beetles and flies have developed glands that secrete substances sought and licked by the termites. The termite nest, because it provides shelter and warmth, may also be occupied by lizards, snakes, scorpions, and some birds.

A few termites, known as inquilinous species, live only in obligatory association with other termite species. Examples of such obligate relationships are Ahamitermes and Incolitermes species, which live only in the mound nests of certain Coptotermes species. In these, the galleries of guests and hosts are completely separate. Inquilinous species feed on the inner carton material of the host nests. Incolitermes, however, depend on the host species not only for food but also for exit holes from the nest during swarming when alates of the inquiline and the host emerge together. Such species’ tolerance is highly unusual. Normally, different species of termites are hostile to one another, and host termites may attack inquilinous guests if partitions between galleries are broken.

Form and function

Reproductives

The primary reproductives in a termite colony are usually one royal pair, a king and queen. They have developed from winged forms (alates) that have flown from a parent colony and shed their wings. Because they spend time outside of the colony on the mating flight, they have hardened, pigmented bodies and large compound eyes. The primary reproductives have several important functions: reproduction, dispersal, and colony formation. In addition, during the initial stages of colony formation, the primary reproductives perform tasks later taken over by the worker caste (e.g., nest construction, housekeeping, care of young).

Termite castes.Encyclopædia Britannica, Inc.

If the king or queen dies, it is replaced by several supplementary reproductives that are slightly pigmented and have either short wing pads (brachypterous) or none (apterous) and reduced compound eyes. These secondary reproductives, which develop from nymphs and may be called neotenics, normally are not present in a colony as long as the primary reproductives remain healthy. If a primary reproductive is lost, a neotenic achieves sexual maturity without attaining a fully winged adult stage or leaving the nest.

The sterile castes are the workers and soldiers. Both are wingless and usually lack eyes. Although these can be either male or female, they lack fully developed reproductive organs. In some species the workers and soldiers are dimorphic (of two sizes), with the larger individuals called major soldiers or workers and the smaller ones called minor soldiers or workers. A few species contain trimorphic soldiers. Most termite species have both soldier and worker castes.

The worker caste usually contains the greatest number of individuals in a colony. Workers are pale in colour, soft-bodied, and have hardened mandibles and mouthparts adapted for chewing. They feed all the other members of the colony (reproductives, soldiers, and young), collect food, groom other colony members, and construct and repair the nest. The worker caste is responsible for the widespread destruction termites can cause. In some primitive termite families a true worker caste is absent, and its functions are carried out by immature individuals called pseudo-workers or pseudergates, which may molt without much change in size.

The primary function of the soldier caste is defense. Since most termite soldiers are blind, they probably locate enemies through tactile and chemical means. The typical termite soldier has a large, dark, hard head. Its long powerful jaws (mandibles) may be hooked and contain teeth. The head and the mandibles are used to defend the colony against predators, usually ants. The attacking soldier makes rapid lunging movements, opening and closing its mandibles in a scissorlike action that can behead, dismember, lacerate, or grip a foe. In some soldiers (e.g., Capritermes) the mandibles are of an asymmetrical, snapping type, with the left mandible twisted and arched and the right bladelike. In defense, the mandibles lock together and release with a loud click, like the snapping of fingers. Some soldiers (e.g., Cryptotermes) use their heads, which are short and truncated in front (phragmotic), to plug the entrance holes of nests.

The higher termites (Termitidae) may supplement or replace mandibular defenses with chemical mechanisms that utilize sticky, possibly toxic, liquids secreted by either the salivary or the frontal glands. The whitish or brownish liquid becomes rubberlike after exposure to air and entangles enemies. The frontal gland of some termites (e.g., Coptotermes and Rhinotermes) occupies a large portion of the abdominal cavity and opens by means of a frontal pore (fontanelle), through which the liquid is ejected. The liquid from the frontal pore of the minor soldier of Rhinotermes flows down a groove in the elongated labrum to its hairy tip, where it volatilizes as a repellent gas.

The mandibles of soldiers with exclusively chemical defenses (Nasutitermitinae) have become reduced in size and are nonfunctional. In these, the head has become elongated into a long snout (nasus), and the frontal gland, which occupies a major portion of the head, opens at the end of the snout. These nasute soldiers can accurately fire a clear, sticky, resinous liquid for many centimetres. A few genera lack a soldier caste, and the mechanisms for defense in these groups are not known.

Nutrition

The food of termites is mainly cellulose, which is obtained from wood, grass, leaves, humus, manure of herbivorous animals, and materials of vegetative origin (e.g., paper, cardboard, cotton). Most lower termites and many higher ones feed on wood that is either sound or partly decayed. A few termites, known as foragers or harvesters, collect and eat grass, leaves, and straw. Many higher termites (family Termitidae) are humivores, or exclusively humus feeders.

As with other social insects, not all members of a termite colony feed directly. Because reproductives, soldiers, and young nymphs in lower families (all nymphs in Termitidae) cannot feed themselves directly, they must be fed by workers. Workers, or in families without them, the older nymphs, feed for the entire colony and transfer food to dependent castes either by mouth feeding or by anal feeding. Food transferred by mouth may consist of either pastelike regurgitated chewed wood and saliva or a clear liquid. This method is used in all termite families. During anal feeding, present only among lower termites, a pastelike liquid or droplet is discharged from the anus of the worker and licked away by the dependent castes. This liquid food, distinct from feces, consists of hindgut fluid containing protozoans, products of digestion, and wood fragments.

Cellulose digestion in lower termite families depends upon symbioticflagellate protozoa, which live anaerobically (without oxygen) in the termite hindgut and secrete enzymes (cellulase and cellobiase) that break down cellulose into a simple sugar (glucose) and acetic acid. The termites depend entirely on protozoans for cellulose digestion and would starve without them. Newly hatched nymphs acquire protozoa from older, infected termites during anal feeding, a type of feeding necessary to lower termites that harbour protozoans.

Since the protozoans lost at the time of each molt are reacquired only through anal feeding, termites live in groups that allow contact of molting nymphs with infected, nonmolting individuals. It is possible that the necessity for transfer of protozoans was responsible for the evolution of the termite society.

Higher termites lack symbiotic protozoans, and only bacteria are present in the gut. Digestion may occur with the aid of bacterial cellulase and cellobiase enzymes, but in some species the termites themselves may secrete the enzymes.

In addition to cellulose, termites require vitamins and nitrogenous foods (e.g., proteins), which probably are supplied by fungi normally present in the decayed wood diet common to most termites. The fungi also may break down wood into components that are easily digested by termites.

Fungus gardens

The Macrotermitinae (family Termitidae) cultivate symbiotic fungi (Termitomyces). The termites construct spongelike “fungus gardens,” or combs, possibly of fecal matter rich in the carbohydrate lignin. The fungi grow on the combs, and the termites consume both fungi and combs. The fungi break down the fecal matter used to construct the combs into substances that can be reutilized by the termites. Nitrogen other than that from fungi is supplied by controlled cannibalism. The termites consume cast-off skins and dead, injured, and excess members of the colony.

Among the members of a termite colony there is continuous exchange of information, such as alarm, indication of direction and presence of a food source, and, among reproductives, calling and pairing behaviour. Information is communicated mainly by vibrations, physical contact, and chemical signals (e.g., odour). Visual cues may be used by individuals outside of the colony where light is present, but they play no role in the dark colony interior.

Many termite species leave their nests to forage for food. Workers (or older nymphs) and soldiers march in columns along the ground and carry grass, pine needles, and seeds for storage in the nest. The foraging trail between the nest and the food source may be indicated by deposits of fecal matter, covered runways over the trail, or pheromones secreted by a sternal gland as the termite drags its abdomen along the ground. The pheromone odour is detected by other termites through olfactory receptors.

Termites communicate alarm by vibrations, odour, and physical contact. Alarmed termites may tap their heads against the ground, quiver and jerk, or run in a zigzag fashion, bumping into other individuals. Although the vertical head-tapping movements produce rattling sounds audible to the human ear, termites cannot hear airborne sounds. It is the substratum vibration that they sense through the vibratory receptors located on their legs. The zigzag and horizontal jerking movements communicate alarm by contact; as an alarmed termite bumps into other termites, they, too, become alarmed. During this excitatory running, the alarmed termite leaves a scent trail, similar to the foraging trail, of pheromone that communicates direction and serves to recruit workers and soldiers to the point of disturbance.

Evolution, paleontology, and classification

Termites are related to the roaches and probably have evolved from a primitive roachlike ancestor. The most primitive living roach, the subsocial, wood-eating Cryptocercus punctulatus, which lives in rotten logs, has affinities with the termites. Cryptocercus harbours symbiotic, cellulose-digesting protozoans of the same genera as those found in the hindgut of primitive termites. The genitalia and certain internal structures of Cryptocercus have basic anatomic resemblances to those of the most primitive living termite, Mastotermes darwiniensis, from Australia. Mastotermes has further affinities with other roaches: its hind wing has a folded anal lobe, and its eggs are not laid singly as those of other termites but in clusters held together by a gelatinous material resembling the egg case of roaches.

Evidence of the relationship to primitive roaches suggests that termites evolved in the Late Permian (approximately 251,000,000 years ago), although the known fossil termites date only from the Early Cretaceous (about 130,000,000 years ago). The termite social system may be older than any other society. Ant social systems are estimated to be only 100,000,000 years old.

Classification

Termites of the order Isoptera are small to medium-sized insects that live in social groups, or colonies, and are characterized by their highly developed caste system. The mouthparts are modified for chewing. Antennae are moniliform (beadlike) or filiform (threadlike). Isopterans are very soft-bodied insects, usually light in colour. Head structures and the presence or absence of individual caste members are used to distinguish termite families.

Termites, although often called white ants, differ from hymenopterans (bees, ants, and wasps) in several ways. Termites have a hemimetabolous (gradual) metamorphosis and pass through a series of nymphal stages. Hymenopterans have the more common holometabolous metamorphosis, with distinct larval, pupal, and adult stages. Termite social castes (reproductives, sterile workers, and sterile soldiers) usually contain members of both sexes in equal numbers, and both males and females develop from fertilized eggs. In the hymenopteran colony, however, the sterile castes contain females only, with both sterile and reproductive females developing from fertilized eggs. Reproductive males develop by parthenogenesis from unfertilized eggs. The thorax in termites is joined broadly to the abdomen, without the “waist” characteristic of bees, ants, and wasps. Termites have two pairs of membranous wings, nearly equal in size, that break along a suture when shed, leaving only the wing base, or “scale,” attached to the thorax. This is probably the most distinguishing characteristic of isopterans.

Thirty living, 13 fossil species (1, the earliest known termite fossil, from Lower Cretaceous, Labrador); includes rotten-wood termites and harvester termites that forage and store food in nests; Zootermopsis, largest termite in North America, found in Rocky Mountains at altitudes of 2,000 to 2,500 metres; Archotermopsis, found in Himalayas; Hodotermes species, serious pests of African grasslands.

Family Rhinotermitidae (subterranean termites)

Lives under damp conditions; 158 living, 13 fossil species; Reticulitermes, widely distributed in North America and other temperate and subtemperate regions and a serious pest; Coptotermes, a serious pest in tropical and subtropical regions.

Family Serritermitidae

One living species in South America; specialized family evolved from Rhinotermitidae.